There has been a steadily increasing growth in the number of personal devices a typical user owns, carries or wears. Devices such as smartphones, tablets and laptops have been at constant risk of being left unattended thereby putting at risk the personal data on those devices being stolen. A user's proximity to these devices is a strong indication of whether these devices are in the user's physical control. With the increasing use of wearable devices such as smart watches, fitness trackers, and smart glasses, it is possible to exploit their proximity to the other devices (e.g., smart phones, tablets, laptops, cars, etc.) for user authentication.
Present techniques that use proximity for security or authentication provide only a low level of control because they rely on Received Signal Strength (RSS) which is unreliable for authentication purposes. In addition to the security and authentication issues caused by inaccurately detected distance between phone and wearable devices, attackers can also conduct spoofing attacks to gain access to another user's smart phone. Other acoustic-based methods have drawbacks, such as being vulnerable to man-in-the-middle attacks and are not suitable for applications with high security requirements. Other methods are more secure but have inadequate distance measurements between devices or are not able to detect distances in real time.